1. Field of the Invention
This invention relates to a crosslinkable aqueous polyester emulsion and a process for preparing the same. In particular, it relates to an emulsion polymerization of a crosslinkable aqueous polyester and an acrylic monomer hybrid. The blends of these emulsions, after being formed into films, have excellent water resistance and adhesion, and are thus useful in printing on Nylon, PET, and OPP films, as well as other high performance industrial surface coating and adhesion applications.
2. Description of Related Art
In order to improve water-resistance and block resistance of an emulsion suitably for use in industrial surface coating and adhesive applications, a water dispersible sulfo-polyester and an acrylic resin blends are prepared. For example, U.S. Pat. Nos. 4,921,899, 5,218,032, and 5,294,650 disclose emulsion containing blends of water dispersible sulfo-polyester and acrylic resin, and the sulfo-polyester contains at least 12 mole percent of difunctional sulfomonomer. Since the blends of these emulsions contain a large amount of hydrophilic groups, these kinds of emulsions, after being formed into films, have poor water resistance.
As an alternative, emulsion polymerization of an acrylic monomer or ethylene monomer, using a sulfonate group-containing aqueous polyester as an emulsifier are proposed to improve the water resistance and adhesion on certain substrates. In this approach, the water-soluble or water-dispersible sulfo-polyester must have low molecular weight and the monomers must be dissoluble or slightly soluble in water. For example, U.S. Pat. No. 4,946,932 discloses a method of preparing emulsion by emulsion polymerization of a water dispersible sulfonate group-containing polyester and unsaturated monomers with the addition of aqueous initiators. Moreover, in order to improve the emulsion stability, it is proposed in U.S. Pat. No. 5,268,421 to use an additional reactive surfactant in the preparation of the blends. Since the polyester used in the preparation of the emulsion must contain enough sulfonate groups to provide appropriate water dispersion properties, the emulsion, after being formed into a film, exhibits poor water-resistance.
U.S. Pat. No. 5,342,877 discloses an emulsion which is prepared from a water-soluble polyester, a styrene monomer, and a crosslinkable containing acrylic monomer blends. However, since the water-soluble polyester still contains many high hydrophilic sulfonate groups, the water-resistance of the emulsion, after a film is formed, is not much improved.